U.S. patent application number 13/735602 was filed with the patent office on 2013-07-25 for thick film silver paste containing copper and lead-tellurium-oxide and its use in the manufacture of semiconductor devices.
This patent application is currently assigned to E I DU PONT DE NEMOURS AND COMPANY. The applicant listed for this patent is E I DU PONT DE NEMOURS AND COMPANY. Invention is credited to RAJ G. RAJENDRAN.
Application Number | 20130187101 13/735602 |
Document ID | / |
Family ID | 48742485 |
Filed Date | 2013-07-25 |
United States Patent
Application |
20130187101 |
Kind Code |
A1 |
RAJENDRAN; RAJ G. |
July 25, 2013 |
THICK FILM SILVER PASTE CONTAINING COPPER AND LEAD-TELLURIUM-OXIDE
AND ITS USE IN THE MANUFACTURE OF SEMICONDUCTOR DEVICES
Abstract
The present invention is directed to a thick film silver paste
comprising (i) silver, (ii) copper, and (iii) a Pb--Te--O all
dispersed in an organic medium. The present invention is further
directed to an electrode formed from the paste and a semiconductor
device and, in particular, a solar cell comprising such an
electrode. The electrodes provide good electrical performance.
Inventors: |
RAJENDRAN; RAJ G.;
(Hockessin, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
E I DU PONT DE NEMOURS AND COMPANY; |
Wilmington |
DE |
US |
|
|
Assignee: |
E I DU PONT DE NEMOURS AND
COMPANY
Wilmington
DE
|
Family ID: |
48742485 |
Appl. No.: |
13/735602 |
Filed: |
January 7, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61589895 |
Jan 24, 2012 |
|
|
|
Current U.S.
Class: |
252/514 |
Current CPC
Class: |
Y02E 10/50 20130101;
Y02E 10/547 20130101; H01L 31/0224 20130101; H01B 1/22 20130101;
H01L 31/022425 20130101; H01L 31/068 20130101 |
Class at
Publication: |
252/514 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224 |
Claims
1. A thick film silver paste comprising: (a) silver; (b) copper;
(c) Pb--Te--O; and (d) organic medium; wherein said silver, said
copper, and said Pb--Te--O are dispersed in said organic
medium.
2. The thick film silver paste of claim 1, wherein the total
content of said silver, said copper and said Pb--Te--O is 50 to 95
wt %, based on the total weight of said thick film silver
paste.
3. The thick film silver paste of claim 2, said thick film silver
paste comprising 50-95 vol % silver and 5-50 vol % copper, wherein
said vol % are based on the total volume of said silver and said
copper.
4. The thick film silver paste of claim 3, said thick film silver
paste comprising 65-95 vol % silver and 5-35 vol % copper, wherein
said vol % are based on the total volume of said silver and said
copper.
5. The thick film silver paste of claim 1, wherein said copper is
surface coated copper selected from the group consisting of fatty
acid coated copper, tin coated copper, silver coated copper and
mixtures thereof.
6. The thick film silver paste of claim 1, said thick film silver
paste comprising 0.5-10 wt % Pb--Te--O, wherein said wt % is based
on the total weight of said thick film silver paste.
7. The thick film silver paste of claim 6, wherein said Pb--Te--O
is a Pb--Te--Li--O.
8. The thick film silver paste of claim 6, wherein said Pb--Te--O
is a Pb--Te--Li--Ti--O.
9. The thick film silver paste of claim 6, wherein said Pb--Te--O
is a Pb--Te--Li--B--Bi--O.
10. The thick film silver paste of claim 6, said thick film silver
paste comprising 0.5-5 wt % Pb--Te--O, wherein said wt % is based
on the total weight of said thick film silver paste.
11. A solar cell comprising an electrode formed from a thick film
silver paste comprising: a) silver; b) copper; c) Pb--Te--O; and d)
organic medium; wherein said silver, said copper, and said
Pb--Te--O are dispersed in said organic medium and wherein said
thick film silver paste has been fired to remove the organic medium
and form said electrode.
12. The solar cell of claim 11, wherein the total content of said
silver, said copper and said Pb--Te--O is 50 to 95 wt %, based on
the total weight of said thick film silver paste.
13. The solar cell of claim 12, said thick film silver paste
comprising 50-95 vol % silver and 5-50 vol % copper, wherein said
vol % are based on the total volume of said silver and said
copper.
14. The solar cell of claim 13, said thick film silver paste
comprising 65-95 vol % silver and 5-35 vol % copper, wherein said
vol % are based on the total volume of said silver and said
copper.
15. The solar cell of claim 11, wherein said copper is surface
coated copper selected from the group consisting of fatty acid
coated copper, tin coated copper, silver coated copper and mixtures
thereof.
16. The solar cell of claim 11, said thick film silver paste
comprising 0.5-10 wt % Pb--Te--O, wherein said wt % is based on the
total weight of said thick film silver paste.
17. The solar cell of claim 16, wherein said Pb--Te--O is a
Pb--Te--Li--O.
18. The solar cell of claim 16, wherein said Pb--Te--O is a
Pb--Te--Li--Ti--O.
19. The solar cell of claim 16, wherein said Pb--Te--O is a
Pb--Te--Li--B--Bi--O.
20. The solar cell of claim 11, wherein said copper is surface
coated copper and said Pb--Te--O is a Pb--Te--Li--O.
Description
FIELD OF THE INVENTION
[0001] The present invention is directed primarily to a thick film
silver paste and electrodes formed from the thick film silver
paste. It is further directed to a silicon semiconductor device
and, in particular, it pertains to the use of the thick film paste
in the formation of an electrode of a solar cell.
TECHNICAL BACKGROUND OF THE INVENTION
[0002] A conventional solar cell structure with a p-type base has a
negative electrode that is typically on the front-side or sun side
of the cell and a positive electrode on the back side. Radiation of
an appropriate wavelength falling on a p-n junction of a
semiconductor body serves as a source of external energy to
generate electron-hole pairs in that body. Because of the potential
difference which exists at a p-n junction, holes and electrons move
across the junction in opposite directions and thereby give rise to
a flow of electric current that is capable of delivering power to
an external circuit. Most solar cells are in the form of a silicon
wafer that has been metallized, i.e., provided with metal
electrodes that are electrically conductive. Typically thick film
pastes or inks (referred to simply as "pastes" hereafter) are
screen-printed onto the substrate and fired to form the
electrodes.
[0003] The front or sun side of the silicon wafer is often coated
with an anti-reflective coating (ARC) to prevent reflective loss of
incoming sunlight, thus increasing the efficiency of the solar
cell. Typically, a two-dimensional electrode grid pattern, i.e.
"front electrode," makes a connection to the n-side of the silicon,
and a coating of aluminum on the opposite side (back electrode)
makes connection to the p-side of the silicon. These contacts are
the electrical outlets from the p-n junction to the outside
load.
[0004] The front electrodes of silicon solar cells are generally
formed by screen-printing a paste. Typically, the paste contains
electrically conductive particles, glass frit and an organic
medium. After screen-printing, the wafer and paste are dried at
150.degree. C. for a few minutes and then fired in air, typically
at furnace setpoint temperatures of about 656-1000.degree. C. for a
few seconds to form a dense solid of electrically conductive
traces. The organic components are burned away in this firing step.
Also during this firing step, the glass frit and any added flux
reacts with and etches through the anti-reflective coating and
facilitates the formation of intimate silicon-electrode contact.
The glass frit and any added flux also provide adhesion to the
substrate and aid in the adhesion of subsequently soldered leads to
the electrode. Good adhesion to the substrate and high solder
adhesion of the leads to the electrode are important to the
performance of the solar cell as well as the manufacturability and
reliability of the solar modules.
[0005] There is an on-going effort to provide paste compositions
that result in electrodes with reduced silver content while
maintaining electrical performance.
SUMMARY OF THE INVENTION
[0006] The present invention provides a thick film silver paste
comprising: [0007] (a) silver; [0008] (b) copper; [0009] (c)
Pb--Te--O; and [0010] (d) organic medium; wherein the silver, the
copper, and the Pb--Te--O are dispersed in the organic medium.
[0011] The thick film silver paste contains 50 to 95 wt % inorganic
solids, i.e., the total content of the silver, the copper and the
Pb--Te--O is 50 to 95 wt %, based on the total weight of the
paste.
[0012] The invention also provides a semiconductor device, and in
particular, a solar cell, comprising an electrode formed from the
instant thick film paste, wherein the thick film paste has been
fired to remove the organic medium and form the electrode. In an
embodiment the electrode is a front electrode. In another
embodiment the electrode is a back electrode, e.g., a tabbing
electrode.
BRIEF DESCRIPTION OF THE FIGURES
[0013] FIGS. 1A-1F illustrate the fabrication of a semiconductor
device. Reference numerals shown in FIG. 1 are explained below.
[0014] 10: p-type silicon substrate [0015] 20: n-type diffusion
layer [0016] 30: ARC (e.g., silicon nitride film, titanium oxide
film, or silicon oxide film) [0017] 40: p+ layer (back surface
field, BCF) [0018] 60: aluminum paste deposited on back side [0019]
61: aluminum back side electrode (obtained by firing back side
aluminum paste) [0020] 70: silver/aluminum paste deposited on back
side [0021] 71: silver/aluminum back side electrode (obtained by
firing back side silver/aluminum paste) [0022] 500: paste of the
instant invention deposited on front side [0023] 501: front
electrode (formed by firing front side paste 500)
DETAILED DESCRIPTION OF THE INVENTION
[0024] The thick film silver paste of the instant invention
simultaneously provides the ability to form an electrode wherein
the electrode has reduced cost because of the reduced amount of
silver used but also exhibits good electrical and improved adhesion
properties. The thick film silver paste can be printed or applied
with the desired pattern, such as by screen-printing, plating,
ink-jet printing, extrusion, shaped or multiple printing, or
ribbons.
[0025] The thick film silver paste comprises silver, copper,
Pb--Te--O, and an organic medium. In one embodiment, the thick film
paste comprises 50-95 vol % silver and 5-50 vol % copper, wherein
the vol % are based on the total volume of the silver and the
copper.
[0026] The thick film silver paste of the instant invention
provides electrodes with improved adhesion and reduced silver
content while maintaining electrical performance.
[0027] Each constituent of the thick film silver paste of the
present invention is discussed in detail below.
Silver
[0028] The silver (Ag) can be in the form of silver metal, alloys
of silver, or mixtures thereof. Typically, in a silver powder, the
silver particles are in a flake form, a spherical form, a granular
form, a crystalline form, other irregular forms and mixtures
thereof. The silver can be provided in a colloidal suspension. The
silver can also be in the form of silver resonates (organometallic
silver), silver oxide (Ag.sub.2O), silver salts such as AgCl,
AgNO.sub.3, AgOOCCH.sub.3 (silver acetate), AgOOCF.sub.3 (silver
trifluoroacetate), silver orthophosphate (Ag.sub.3PO.sub.4), or
mixtures thereof. Other forms of silver compatible with the other
constituents can also be used.
[0029] In one embodiment, the thick film paste comprises 50-95 vol
% silver, wherein the vol % is based on the total volume of the
silver and the copper. In another embodiment, the thick film paste
comprises 70-90 vol % silver, wherein the vol % is based on the
total volume of the silver and the copper. In still another
embodiment, the thick film paste comprises 50-60 vol % silver,
wherein the vol % is based on the total volume of the silver and
the copper.
[0030] In one embodiment, the thick film paste comprises coated
silver particles that are electrically conductive. Suitable
coatings include surfactants and phosphorous-containing compounds.
Suitable surfactants include polyethyleneoxide, polyethyleneglycol,
benzotriazole, poly(ethyleneglycol)acetic acid, lauric acid, oleic
acid, capric acid, myristic acid, linolic acid, stearic acid,
palmitic acid, stearate salts, palmitate salts, and mixtures
thereof. The salt counter-ions can be ammonium, sodium, potassium,
and mixtures thereof.
[0031] The particle size of the silver is not subject to any
particular limitation. In one embodiment, the average particle size
is less than 15 .mu.m; in another embodiment, the average particle
size is in the range of 1 to 6 .mu.m.
Copper
[0032] The copper (Cu) is in the form of a powder with granular
particles. Typically, the average particle size is less than 15
.mu.m. In an embodiment, the particle size distribution of the
powder is such that the average particle size is between 3 and 15
.mu.m.
[0033] In an embodiment, the copper particles are surface coated
copper particles. The surface coating material may include
electrically conductive materials or thermally decomposable
non-conductive materials. In one such embodiment, the copper
particles are fatty acid coated copper particles. Examples of
suitable fatty acids include lauric acid, oleic acid, capric acid,
myristic acid, linolic acid, stearic acid and mixtures thereof. In
another such embodiment, the copper particles are tin-coated copper
particles. In still another such embodiment, the copper particles
are silver-coated copper particles.
[0034] In an embodiment, the thick film paste comprises 5-50 vol %
copper, wherein the vol % is based on the total volume of the
silver and the copper. In another embodiment, the thick film paste
comprises 10-30 vol % copper, wherein the vol % is based on the
total volume of the silver and the copper, In still another
embodiment, the thick film paste comprises 40-50 vol % copper,
wherein the vol % is based on the total volume of the silver and
the copper.
Lead-Tellurium-Oxide
[0035] A component of the paste is a lead-tellurium-oxide
(Pb--Te--O). In an embodiment, this oxide is a glass composition,
e.g., a glass frit. In a further embodiment, this oxide is
crystalline, partially crystalline, amorphous, partially amorphous,
or combinations thereof. In an embodiment, the Pb--Te--O may
include more than one glass composition. In an embodiment, the
Pb--Te--O composition may include a glass composition and an
additional composition, such as a crystalline composition.
[0036] The lead-tellurium-oxide (Pb--Te--O) is prepared by mixing
TeO.sub.2, lead oxide and other oxides to be incorporated therein
(or other materials that decompose into the desired oxides when
heated) using techniques understood by one of ordinary skill in the
art. The lead oxide may include one or more components selected
from the group consisting of PbO, Pb.sub.3O.sub.4, and PbO.sub.7.
Such preparation techniques may involve heating the mixture in air
or an oxygen-containing atmosphere to form a melt, quenching the
melt, and grinding, milling, and/or screening the quenched material
to provide a powder with the desired particle size. Melting the
mixture of lead, tellurium and other oxides to be incorporated
therein is typically conducted to a peak temperature of 800 to
1200.degree. C. The molten mixture can be quenched, for example, on
a stainless steel platen or between counter-rotating stainless
steel rollers to form a platelet. The resulting platelet can be
milled to form a powder. Typically, the milled powder has a
d.sub.50 of 0.1 to 3.0 microns. One skilled in the art of producing
glass frit may employ alternative synthesis techniques such as but
not limited to water quenching, sol-gel, spray pyrolysis, or others
appropriate for making powder forms of glass.
[0037] The oxide product of the above process is typically
essentially an amorphous (non-crystalline) solid material, i.e., a
glass. However, in some embodiments the resulting oxide is
amorphous, partially amorphous, partially crystalline, crystalline
or combinations thereof. As used herein "glass frit" includes all
such products.
[0038] Glass compositions, also termed glass frits, are described
herein as including percentages of certain components.
Specifically, the percentages are the percentages of the components
used in the starting material that was subsequently processed as
described herein to form a glass composition. Such nomenclature is
conventional to one of skill in the art. In other words, the
composition contains certain components, and the percentages of
those components are expressed as a percentage of the corresponding
oxide form. As recognized by one of ordinary skill in the art in
glass chemistry, a certain portion of volatile species is released
during the process of making the glass. An example of a volatile
species is oxygen. It should also be recognized that while the
glass behaves as an amorphous material it will likely contain minor
portions of a crystalline material.
[0039] If starting with a fired glass, one of ordinary skill in the
art may calculate the percentages of starting components described
herein using methods known to one of skill in the art including,
but not limited to: Inductively Coupled Plasma-Mass Spectroscopy
(ICP-MS), Inductively Coupled Plasma-Atomic Emission Spectroscopy
(ICP-AES), and the like. In addition, the following exemplary
technique is used: X-Ray Fluorescence spectroscopy (XRF); Nuclear
Magnetic Resonance spectroscopy (NMR); Electron Paramagnetic
Resonance spectroscopy (EPR); Mossbauer spectroscopy; electron
microprobe Energy Dispersive Spectroscopy (EDS); electron
microprobe Wavelength Dispersive Spectroscopy (WDS); or
Cathodo-Luminescence (CL).
[0040] One of ordinary skill in the art would recognize that the
choice of raw materials could unintentionally include impurities
that are incorporated into the glass during processing. For
example, the impurities is present in the range of hundreds to
thousands ppm. The presence of the impurities would not alter the
properties of the glass, the composition, e.g. a thick film
composition, or the fired device. For example, a solar cell
containing a thick film composition may have the efficiency
described herein, even if the thick film composition includes
impurities.
[0041] Typically, the mixture of PbO and TeO.sub.2 powders used to
make the Pb--Te--O includes 5 to 95 mol % of lead oxide and 5 to 95
mol % of tellurium oxide, based on the combined powders. In one
embodiment, the mixture of PbO and TeO.sub.2 powders includes 25 to
85 mol % of lead oxide and 15 to 75 mol % of tellurium oxide, based
on the combined powders. In another embodiment, the mixture of PbO
and TeO.sub.2 powders includes 25 to 65 mol % of lead oxide and 35
to 75 mol % of tellurium oxide, based on the combined powders.
[0042] In one embodiment, the electrically conductive paste
comprises 0.5-10 wt % Pb--Te--O, wherein the wt % is based on the
total weight of the thick film paste. In another embodiment, the
thick film paste comprises 0.5-5 wt % Pb--Te--O, wherein the wt %
is based on the total weight of the composition.
[0043] In some embodiments, the mixture of PbO and TeO.sub.2
powders further includes one or more other metal compounds.
Suitable other metal compounds include TiO.sub.2, LiO.sub.2,
B.sub.2O.sub.3, PbF.sub.2, SiO.sub.2, Na.sub.2O, K.sub.2O,
Rb.sub.2O, Cs.sub.2O, Al.sub.2O.sub.3, MgO, CaO, SrO, BaO,
V.sub.2O.sub.5, ZrO.sub.2, MoO.sub.3, Mn.sub.2O.sub.3, Ag.sub.2O,
ZnO, Ga.sub.2O.sub.3, GeO.sub.2, In.sub.2O.sub.3, SnO.sub.2,
Sb.sub.2O.sub.3, Bi.sub.2O.sub.3, BiF.sub.3, P.sub.2O.sub.5, CuO,
NiO, Cr.sub.2O.sub.3, Fe.sub.2O.sub.3, CoO, Co.sub.2O.sub.3, and
CeO.sub.2.
[0044] Table 1 lists some examples of powder mixtures containing
PbO, TeO.sub.2 and other optional metal compounds that can be used
to make lead-tellurium oxides. This list is meant to be
illustrative, not limiting. In Table 1, the amounts of the
compounds are shown as wt %, based on the weight of the total glass
composition.
[0045] Therefore as used herein, the term "Pb--Te--O" also includes
metal oxides that contain oxides of one or more elements selected
from the group consisting of Si, Sn, Li, Ti, B, Ag, Na, K, Rb, Cs,
Ge, Ga, In, Ni, Zn, Ca, Mg, Sr, Ba, Se, Mo, W, Y, As, La, Nd, Co,
Pr, Gd, Sm, Dy, Eu, Ho, Yb, Lu, Bi, Ta, V, Fe, Hf, Cr, Cd, Sb, F,
Zr, Mn, P, Cu, Ce, and Nb.
TABLE-US-00001 TABLE 1 Powder Wt % Wt % Wt % Wt % Wt % Wt % Wt % Wt
% Wt % mixture PbO TeO.sub.2 PbF.sub.2 SiO.sub.2 B.sub.2O.sub.3
P.sub.2O.sub.5 SnO.sub.2 Ag.sub.2O Li.sub.2O A 32.95 67.05 B 38.23
51.26 10.50 C 67.72 32.28 D 72.20 27.80 E 80.75 19.25 F 59.89 9.30
16.19 14.82 G 75.86 9.26 14.88 H 48.06 51.55 0.39 I 48.16 51.65
0.19 J 47.44 50.88 1.68 K 47.85 51.33 0.82 L 41.76 44.80 0.32 0.80
12.32 M 46.71 50.10 3.19 N 46.41 49.78 3.80 O 45.11 48.39 6.50 P
44.53 47.76 7.71 Q 48.05 51.54 0.41 R 47.85 51.33 0.82 S 47.26
50.70 2.04 T 45.82 49.19 U 48.04 51.53 V 39.53 28.26 W 48.04 51.53
0.42
[0046] In one embodiment, the Pb--Te--O includes boron, i.e., the
Pb--Te--O is Pb--Te--B--O. The starting mixture used to make the
Pb--Te--B--O includes (based on the weight of the total starting
mixture) PbO that is 25 to 75 wt %, 30 to 60 wt %, or 30 to 50 wt
%; TeO.sub.2 that is 10 to 70 wt %, 25 to 60 wt %, or 40 to 60 wt
%; B.sub.2O.sub.3 that is 0.1 to 15 wt %, 0.25 to 5 wt %, or 0.4 to
2 wt %.
[0047] In an embodiment, PbO, TeO.sub.2, and B.sub.2O.sub.3 are
80-100 wt % of the Pb--Te--B--O composition. In a further
embodiment, PbO, TeO.sub.2, and B.sub.2O.sub.3 are 85-100 wt % or
90-100 wt % of the Pb--Te--B--O composition.
[0048] In a further embodiment, in addition to the above PbO,
TeO.sub.2, and B.sub.2O.sub.3, the starting mixture used to make
the Pb--Te--B--O includes one or more of PbF.sub.2, SiO.sub.2,
BiF.sub.3, SnO.sub.2, Li.sub.2O, Bi.sub.2O.sub.3, ZnO,
V.sub.2O.sub.5, Na.sub.2O, TiO.sub.2, Al.sub.2O.sub.3, CuO,
ZrO.sub.2, CeO.sub.2, or Ag.sub.2O. In an embodiment, one or more
of these components are 0.1-20 wt %, 0.1-15 wt %, or 0.1-10 wt % of
the Pb--Te--B--O composition. In aspects of this embodiment (based
on the weight of the total starting mixture):
[0049] the PbF.sub.2 is 0.1 to 20 wt %, 0.1 to 15 wt %, or 5 to 10
wt %;
[0050] the SiO.sub.2 is 0.1 to 11 wt %, 0.1 to 5 wt %, 0.25 to 4 wt
%, or 0.1 to 0.5 wt %;
[0051] the BiF.sub.3 is 0.1 to 15 wt %, 0.1 to 10 wt %, or 1 to 10
wt %;
[0052] the SnO.sub.2 is 0.1 to 5 wt %, 0.1 to 2 wt %, or 0.5 to 1.5
wt %;
[0053] the ZnO is 0.1 to 5 wt %, 0.1 to 3 wt %, or 2 to 3 wt %;
[0054] the V.sub.2O.sub.5 is 0.1 to 5 wt %, 0.1 to 1 wt %, or 0.5
to 1 wt %;
[0055] the Na.sub.2O is 0.1 to 5 wt %, 0.1 to 3 wt %, or 0.1 to 1.5
wt %;
[0056] the CuO is 0.1 to 5 wt %, 0.1 to 3 wt %, or 2 to 3 wt %;
[0057] the ZrO.sub.2 is 0.1 to 3 wt %, 0.1 to 2 wt %, or 0.1 to 1
wt %;
[0058] the CeO.sub.2 is 0.1 to 5 wt %, 0.1 to 3 wt %, or 0.1 to 2.5
wt %;
[0059] the Li.sub.2O is 0.1 to 5 wt %, 0.1 to 3 wt %, or 0.25 to 2
wt %;
[0060] the Bi.sub.2O.sub.3 is 0.1 to 15 wt %, 0.1 to 10 wt %, or 5
to 8 wt %;
[0061] the TiO.sub.2 is 0.1 to 5 wt %, 0.25 to 5 wt %, or 0.25 to
2.5 wt %;
[0062] the Al.sub.2O.sub.3 is 0.1 to 3 wt %, 0.1 to 2.5 wt %, or
0.1 to 2 wt %; and
[0063] the Ag.sub.2O is 0.1 to 10 wt %, 1 to 10 wt %, or 1 to 8 wt
%.
[0064] In an embodiment, the Pb--Te--B--O is a homogenous powder.
In a further embodiment, the Pb--Te--B--O is a combination of more
than one powder, wherein each powder may separately be a homogenous
population. The composition of the overall combination of the
multiple powders is within the ranges described above. For example,
the Pb--Te--B--O may include a combination of two or more different
powders; separately, these powders may have different compositions,
and may or may not be within the ranges described above; however,
the combination of these powders is within the ranges described
above.
[0065] In an embodiment, the Pb--Te--B--O composition may include.
one powder which includes a homogenous powder including some but
not all of the elements of the group Pb, Te, B, and O, and a second
powder, which includes one or more of the elements of the group Pb,
Te, B, and O. For example, the Pb--Te--B--O composition may include
a first powder including Pb, Te, and O, and a second powder
including B.sub.2O.sub.3. In an aspect of this embodiment, the
powders is melted together to form a uniform composition. In a
further aspect of this embodiment, the powder is added separately
to a thick film composition.
[0066] In an embodiment, some or all of the Li.sub.2O is replaced
with Na.sub.2O, K.sub.2O, Cs.sub.2O, or Rb.sub.2O, resulting in a
glass composition with properties similar to the compositions
listed above. In this embodiment, the total alkali metal oxide
content is 0.1 to 5 wt %, 0.1 to 3 wt %, or 0.25 to 3 wt %.
[0067] In a further embodiment, the Pb--Te--B--O composition(s)
herein may include one or more of a third set of components:
GeO.sub.2, Ga.sub.2O.sub.3, In.sub.2O.sub.3, NiO, CoO, ZnO, CaO,
MgO, SrO, MnO, BaO, SeO.sub.2, MoO.sub.3, WO.sub.3, Y.sub.2O.sub.3,
As.sub.2O.sub.3, La.sub.2O.sub.3, Nd.sub.2O.sub.3, Bi.sub.2O.sub.3,
Ta.sub.2O.sub.5, V.sub.2O.sub.5, FeO, HfO.sub.2, Cr.sub.2O.sub.3,
CdO, Sb.sub.2O.sub.3, PbF.sub.2, ZrO.sub.2, Mn.sub.2O.sub.3,
P.sub.2O.sub.5, CuO, Pr.sub.2O.sub.3, Gd.sub.2O.sub.3,
Sm.sub.2O.sub.3, Dy.sub.2O.sub.3, Eu.sub.2O.sub.3, Ho.sub.2O.sub.3,
Yb.sub.2O.sub.3, Lu.sub.2O.sub.3, CeO.sub.2, BiF.sub.3, SnO,
SiO.sub.2, Ag.sub.2O, Nb.sub.2O.sub.5, TiO.sub.2, Rb.sub.2O,
SiO.sub.2, Na.sub.2O, K.sub.2O, Cs.sub.2O, Lu.sub.2O.sub.3,
SnO.sub.2, and metal halides (e.g., NaCl, KBr, Nal, LiF,
ZnF.sub.2).
[0068] Therefore as used herein, the term "Pb--Te--B--O" may also
include metal oxides that contain one or more oxides of elements
selected from the group consisting of Si, Sn, Li, Ti, Ag, Na, K,
Rb, Cs, Ge, Ga, In, Ni, Zn, Ca, Mg, Sr, Ba, Se, Mo, W, Y, As, La,
Nd, Co, Pr, Gd, Sm, Dy, Eu, Ho, Yb, Lu, Bi, Ta, V, Fe, Hf, Cr, Cd,
Sb, F, Zr, Mn, P, Cu, Ce, and Nb.
[0069] In another embodiment, the Pb--Te--O includes lithium, i.e.,
the Pb--Te--O is Pb--Te--Li--O. The starting mixture used to make
the Pb--Te--Li--O includes (based on the weight of the total
starting mixture): [0070] PbO that is 30 to 60 wt %, 40 to 55 wt %,
or 45 to 50 wt %; [0071] TeO.sub.2 that is 40 to 65 wt %, 45 to 60
wt %, or 50 to 55 wt %; and [0072] Li.sub.2O that is 0.1 to 5 wt %,
0.2 to 3 wt %, or 0.3 to 1 wt %.
[0073] In a further embodiment, in addition to the above PbO,
TeO.sub.2, and Li.sub.2O, the starting mixture used to make the
Pb--Te--Li--O includes one or more of SiO.sub.2, SnO.sub.2,
B.sub.2O.sub.3, Ag.sub.2O, BiF.sub.3, V.sub.2O.sub.5, Na.sub.2O,
ZrO.sub.2, TiO.sub.2, CeO.sub.2, Bi.sub.2O.sub.3, Nb.sub.2O.sub.5,
Ta.sub.2O.sub.5, K.sub.2O, MgO, P.sub.2O.sub.5, SeO.sub.2,
Co.sub.3O.sub.4, PdO, RuO.sub.2, NiO, ZnO, CuO, MnO,
Cr.sub.2O.sub.3, or Al.sub.2O.sub.3. In aspects of this embodiment
(based on the weight of the total starting mixture):
[0074] the SiO.sub.2 is 0.1 to 11 wt %. 0.1 to 5 wt %, 0.25 to 4 wt
%, or 0.1 to 0.5 wt %;
[0075] the SnO.sub.2 is 0.1 to 5 wt %, 0.1 to 2 wt %, or 0.5 to 1.5
wt %;
[0076] the B.sub.2O.sub.3 is 0.1 to 10 wt %, 0.1 to 5 wt %, or 0.5
to 5 wt %;
[0077] the Ag.sub.2O is 0.1 to 30 wt %, 0.1 to 20 wt %, 3 to 15 wt
% or 1 to 8 wt %;
[0078] the TiO.sub.2 is 0.1 to 5 wt %, 0.25 to 5 wt %, or 0.25 to
2.5 wt %;
[0079] the PbF.sub.2 is 0.1 to 20 wt %, 0.1 to 15 wt %, or 5 to 10
wt %;
[0080] the BiF.sub.3 is 0.1 to 15 wt %, 0.1 to 10 wt %, or 1 to 10
wt %;
[0081] the ZnO is 0.1 to 5 wt %, 0.1 to 3 wt %, or 2 to 3 wt %;
[0082] the V.sub.2O.sub.5 is 0.1 to 5 wt %, 0.1 to 1 wt %, or 0.5
to 1 wt %;
[0083] the Na.sub.2O is 0.1 to 5 wt %, 0.1 to 3 wt %, or 0.1 to 1.5
wt %;
[0084] the CuO is 0.1 to 5 wt %, 0.1 to 3 wt %, or 2 to 3 wt %;
[0085] the ZrO.sub.2 is 0.1 to 3 wt %, 0.1 to 2 wt %, or 0.1 to 1
wt %;
[0086] the CeO.sub.2 is 0.1 to 5 wt %, 0.1 to 3 wt %, or 0.1 to 2.5
wt %;
[0087] the B.sub.2O.sub.3 is 0.1 to 15 wt %, 0.1 to 10 wt %, or 5
to 8 wt %; and
[0088] the Al.sub.2O.sub.3 is 0.1 to 3 wt %, 0.1 to 2.5 wt %, or
0.1 to 2 wt %.
[0089] In one such embodiment, in addition to the above PbO,
TeO.sub.2, and Li.sub.2O, the starting mixture used to make the
Pb--Te--Li--O includes B.sub.2O.sub.3 and B.sub.2O.sub.3,
BiF.sub.3, or a mixture of Bi.sub.2O.sub.3 and BiF.sub.3. In this
embodiment, the Pb--Te--Li--O is Pb--Te--Li--B--Bi--O.
[0090] In an embodiment, the Pb--Te--Li--O is a homogenous powder.
In a further embodiment, the Pb--Te--Li--O is a combination of more
than one powder, wherein each powder may separately be a homogenous
population. The composition of the overall combination of the two
powders is within the ranges described above. For example, the
Pb--Te--Li--O may include a combination of two or more different
powders; separately, these powders may have different compositions,
and may or may not be within the ranges described above; however,
the combination of these powders is within the ranges described
above.
[0091] In an embodiment, the Pb--Te--Li--O composition may include
one powder which includes a homogenous powder including some but
not all of the elements of the group Pb, Te, Li, and O, and a
second powder, which includes one or more of the elements of the
group Pb, Te, Li, and O.
[0092] In an embodiment, some or all of the Li.sub.2O is replaced
with Na.sub.2O, K.sub.2O, Cs.sub.2O, or Rb.sub.2O, resulting in a
glass composition with properties similar to the compositions
listed above. In this embodiment, the total alkali metal oxide
content is 0.1 to 5 wt %, 0.1 to 3 wt %, or 0.25 to 3 wt %.
[0093] In a further embodiment, the glass frit composition(s)
herein includes one or more of a third set of components:
GeO.sub.2, Ga.sub.2O.sub.3, In.sub.2O.sub.3, NiO, CoO, ZnO, CaO,
MgO, SrO, MnO, BaO, SeO.sub.2, MoO.sub.3, WO.sub.3, Y.sub.2O.sub.3,
As.sub.2O.sub.3, La.sub.2O.sub.3, Nd.sub.2O.sub.3, Bi.sub.2O.sub.3,
B.sub.2O.sub.3, Ta.sub.2O.sub.5, V.sub.2O.sub.5, FeO, HfO.sub.2,
Cr.sub.2O.sub.3, CdO, Sb.sub.2O.sub.3, PbF.sub.2, ZrO.sub.2,
Mn.sub.2O.sub.3, P.sub.2O.sub.5, CuO, La.sub.2O.sub.3,
Gd.sub.2O.sub.3, Sm.sub.2O.sub.3, Dy.sub.2O.sub.3, Eu.sub.2O.sub.3,
Ho.sub.2O.sub.3, Yb.sub.2O.sub.3, Lu.sub.2O.sub.3, CeO.sub.2,
BiF.sub.3, SnO, SiO.sub.2, Ag.sub.2O, Nb.sub.2O.sub.5, TiO.sub.2,
and metal halides (e.g., NaCl, KBr, NaI, LiF).
[0094] Therefore as used herein, the term "Pb--Te--Li--O" includes
metal oxides that contain one or more elements selected from the
group consisting of Si, Sn, Ti, Ag, Na, K, Rb, Cs, Ge, Ga, In, Ni,
Zn, Ca, Mg, Sr, Ba, Se, Mo, W, Y, As, La, Nd, Co, Pr, Gd, Sm, Dy,
Eu, Ho, Yb, Lu, Bi, B, Ta, V, Fe, Hf, Cr, Cd, Sb, F, Zr, Mn, P, Cu,
Ce, and Nb.
[0095] Table 2 lists some examples of powder mixtures containing
PbO, TeO.sub.2, Li.sub.2O that can be used to make
lead-tellurium-lithium-oxides. This list is meant to be
illustrative, not limiting. In Table 2, the amounts of the
compounds are shown as wt %, based on the weight of the total glass
composition.
TABLE-US-00002 TABLE 2 Glass # PbO Li.sub.2O TeO.sub.2 1 48.04 0.42
51.54 2 47.74 1.05 51.21 3 44.73 0.43 54.84 4 55.49 0.41 44.09 5
58.07 0.41 41.52 6 34.51 2.44 63.06 7 42.77 0.43 56.80 8 45.82 4.99
49.19 9 48.04 0.42 51.53 10 47.82 0.89 51.29 11 42.77 0.43 56.80 12
37.31 0.44 62.25 13 57.80 0.86 41.33 14 58.07 0.41 41.52
[0096] In still another embodiment, the Pb--Te--O includes lithium
and titanium, i.e., the Pb--Te--O is Pb--Te--Li--Ti--O. The
starting mixture used to make the Pb--Te--Li--Ti--O includes, based
on the total weight of the starting mixture of the
Pb--Te--Li--Ti--O, 25-65 wt % PbO, 25-70 wt % TeO.sub.2, 0.1-5 wt %
Li.sub.2O and 0.1-5 wt % TiO.sub.2. In one embodiment, the starting
mixture used to make the Pb--Te--Li--Ti--O includes, based on the
total weight of the starting mixture of the Pb--Te--Li--Ti--O,
30-60 wt % PbO, 30-65 wt % TeO.sub.2, 0.25-3 wt % Li.sub.2O and
0.25-5 wt % TiO.sub.2. In another embodiment, the starting mixture
includes 30-50 wt % PbO, 50-65 wt % TeO.sub.2, 0.5-2.5 wt %
Li.sub.2O and 0.5-3 wt % TiO.sub.2.
[0097] In any of the above embodiments, PbO, TeO.sub.2,
Li.sub.2O.sub.3, and TiO.sub.2 is 80-100 wt % of the
Pb--Te--Li--Ti--O composition. In further embodiments, PbO,
TeO.sub.2, Li.sub.2O.sub.3, and TiO.sub.2 is 85-100 wt % or 90-100
wt % of the Pb--Te--Li--Ti--O composition.
[0098] In other embodiments, in addition to the above PbO,
TeO.sub.2, Li.sub.2O, and TiO.sub.2, the Pb--Te--Li--Ti--O further
comprises an oxide selected from the group consisting of SiO.sub.2,
SnO.sub.2, B.sub.2O.sub.3, ZnO, Nb.sub.2O.sub.5, CeO.sub.2,
V.sub.2O.sub.5, Al.sub.2O.sub.3, Ag.sub.2O and mixtures thereof. In
aspects of this embodiment (based on the weight of the total
starting mixture): [0099] the SiO.sub.2 is 0.1 to 10 wt %, 0.1 to 9
wt %, or 2 to 9 wt %; [0100] the SnO.sub.2 is 0.1 to 5 wt %, 0.1 to
4 wt %, or 0.5 to 1.5 wt %; [0101] the B.sub.2O.sub.3 is 0.1 to 10
wt %, 0.1 to 5 wt %, or 1 to 5 wt %; and [0102] the Ag.sub.2O is
0.1 to 30 wt %, 0.1 to 20 wt %, or 3 to 15 wt %.
[0103] In addition, in any of the above embodiments, the glass frit
composition herein may include one or more of a third set of
components: GeO.sub.2, Ga.sub.2O.sub.3, In.sub.2O.sub.3, MO, ZnO,
CaO, MgO, SrO, BaO, SeO.sub.2, MoO.sub.3, WO.sub.3, Y.sub.2O.sub.3,
As.sub.2O.sub.3, La.sub.2O.sub.3, Nd.sub.2O.sub.3, Bi.sub.2O.sub.3,
BiF.sub.3, Ta.sub.2O.sub.5, FeO, HfO.sub.2, Cr.sub.2O.sub.3, CdO,
Sb.sub.2O.sub.3, PbF.sub.2, ZrO.sub.2, Mn.sub.2O.sub.3,
P.sub.2O.sub.5, CuO, Nb.sub.2O.sub.5, Rb.sub.2O, Na.sub.2O,
K.sub.2O, Cs.sub.2O, Lu.sub.2O.sub.3, and metal halides (e.g.,
NaCl, KBr, NaI, LiF, ZnF.sub.2).
[0104] Therefore as used herein, the term "Pb--Te--Li--Ti--O" also
includes oxides containing oxides of one or more elements selected
from the group consisting of Si, Sn, B, Ag, Na, K, Rb, Cs, Ge, Ga,
In, Ni, Zn, Ca, Mg, Sr, Ba, Se, Mo, W, Y, As, La, Nd, Bi, Ta, V,
Fe, Hf, Cr, Cd, Sb, Zr, Mn, P, Cu, Lu, Ce, Al and Nb.
[0105] Tables 3 and 4 list some examples of powder mixtures
containing PbO, TeO.sub.2, and other optional compounds that can be
used to make lead-tellurium-lithium-titanium-oxides. This list is
meant to be illustrative, not limiting. In Tables 3 and 4, the
amounts of the compounds are shown as weight percent, based on the
weight of the total Pb--Te--Li--Ti--O composition.
[0106] The lead-tellurium-lithium-titanium-oxide
(Pb--Te--Li--Ti--O) compositions of Table 3 were prepared by mixing
and blending amounts of Pb.sub.3O.sub.4, TeO.sub.2,
Li.sub.2CO.sub.3, and TiO.sub.2 powders, and optionally, as shown
in Table 3, SiO.sub.2, B.sub.2O.sub.3, Ag.sub.2O, and/or SnO.sub.2
to provide compositions of the oxides with the weight percentages
shown in Table 3, based on the weight of the total glass
composition.
TABLE-US-00003 TABLE 3 Frit SiO.sub.2 PbO B.sub.2O.sub.3 Li.sub.2O
TiO.sub.2 Ag.sub.2O SnO.sub.2 TeO.sub.2 1 8.40 60.90 1.47 0.93 0.70
27.60 2 46.04 0.40 4.18 49.38 3 46.78 0.83 2.22 50.17 4 47.43 0.85
0.84 50.88 5 33.77 2.39 2.13 61.71 6 45.35 0.48 0.43 53.74 7 36.19
1.99 1.77 60.05 8 37.35 2.39 2.13 58.13 9 36.19 1.82 3.06 58.94 10
40.81 2.39 2.13 54.67 11 44.28 0.16 0.42 12.29 42.84 12 40.81 0.59
1.57 9.08 47.95 13 40.81 1.90 1.12 56.16 14 45.77 1.09 0.80 0.71
51.64 15 41.20 0.34 2.30 56.16 16 44.31 0.52 0.46 0.96 3.57 50.17
17 42.92 0.54 0.78 1.31 54.44 18 42.22 0.91 1.53 55.35
[0107] The lead-tellurium-lithium-titanium-oxide
(Pb--Te--Li--Ti--O) compositions of Table 4 were prepared by mixing
and blending amounts of Pb.sub.3O.sub.4, TeO.sub.2,
Li.sub.2CO.sub.3 and TiO.sub.2 powders, and optionally, as shown in
Table 4, B.sub.2O.sub.3, ZnO, Nb.sub.2O.sub.5, CeO.sub.2, and/or
V.sub.2O.sub.5 to provide compositions of the oxides with the
weight percentages shown in Table 4, based on the weight of the
total glass composition.
TABLE-US-00004 TABLE 4 Frit PbO B.sub.2O.sub.3 ZnO Nb.sub.2O.sub.5
Li.sub.2O TiO.sub.2 CeO.sub.2 V.sub.2O.sub.5 TeO2 19 42.27 0.94
1.51 2.87 52.40 20 42.57 4.13 0.92 1.54 50.85 21 45.26 0.86 2.25
0.55 0.49 1.06 49.53
[0108] In one embodiment, the Pb--Te--Li--Ti--O is a homogenous
powder. In a further embodiment, the Pb--Te--Li--Ti--O is a
combination of more than one powder, wherein each powder may
separately be a homogenous population. The composition of the
overall combination of the 2 powders is within the ranges described
above. For example, the Pb--Te--Li--Ti--O may include a combination
of 2 or more different powders; separately, these powders may have
different compositions, and may or may not be within the ranges
described above; however, the combination of these powders is
within the ranges described above.
[0109] In an embodiment, the Pb--Te--Li--Ti--O composition may
include one powder which includes a homogenous powder including
some but not all of the desired elements of the Pb--Te--Li--Ti--O
composition, and a second powder, which includes one or more of the
other desired elements. For example, a Pb--Te--Li--Ti--O
composition may include a first powder including Pb, Te, Li, and O,
and a second powder including TO.sub.2. In an aspect of this
embodiment, the powders is melted together to form a uniform
composition. In a further aspect of this embodiment, the powder is
added separately to a thick film composition.
[0110] In an embodiment, some or all of any Li.sub.2O is replaced
with Na.sub.2O, K.sub.2O, Cs.sub.2O, or Rb.sub.2O, resulting in a
glass composition with properties similar to the compositions
listed above. In this embodiment, the total alkali metal content
will be that described above for Li.sub.2O.
Organic Medium
[0111] The inorganic components of the paste are mixed with an
organic medium to form viscous thick film pastes or less viscous
inks having suitable consistency and rheology for printing. A wide
variety of inert viscous materials can be used as the organic
medium. The organic medium can be one in which the inorganic
components are dispersible with an adequate degree of stability
during manufacturing, shipping and storage of the pastes or inks,
as well as on the printing screen during a screen-printing
process.
[0112] Suitable organic media have rheological properties that
provide stable dispersion of solids, appropriate viscosity and
thixotropy for printing, appropriate wettability of the substrate
and the paste solids, a good drying rate, and good firing
properties. The organic medium can contain thickeners, stabilizers,
surfactants, and/or other common additives. One such thixotropic
thickener is Thixatrol.RTM. (Elementis plc, London, UK). The
organic medium can be a solution of polymer(s) in solvent(s).
Suitable polymers include ethyl cellulose, ethylhydroxyethyl
cellulose, wood rosin, mixtures of ethyl cellulose and phenolic
resins, polymethacrylates of lower alcohols, and the monobutyl
ether of ethylene glycol monoacetate. Suitable solvents include
terpenes such as alpha- or beta-terpineol or mixtures thereof with
other solvents such as kerosene, dibutylphthalate, butyl carbitol,
butyl carbitol acetate, hexylene glycol and alcohols with boiling
points above 150.degree. C., and alcohol esters. Other suitable
organic medium components include: bis(2-(2-butoxyethoxy)ethyl
adipate, dibasic esters such as DBE, DBE-2, DBE-3, DBE-4, DBE-5,
DBE-6, DBE-9, and DBE 1B, octyl epoxy tallate, isotetradecanol, and
pentaerythritol ester of hydrogenated rosin. The organic medium can
also comprise volatile liquids to promote rapid hardening after
application of the paste composition on a substrate.
[0113] The optimal amount of organic medium in the paste is
dependent on the method of applying the composition, the specific
organic medium used and the purpose for which the paste is being
used. The paste contains 5 to 50 wt % of organic medium, based on
the total weight of the composition.
[0114] If the organic medium comprises a polymer, the polymer
typically comprises 8 to 15 wt % of the organic composition.
Preparation of the Paste
[0115] In one embodiment, the paste can be prepared by mixing the
silver, the copper, the Pb--Te--O, and the organic medium in any
order. In some embodiments, the inorganic materials are mixed
first, and they are then added to the organic medium. In other
embodiments, the silver, which is the major portion of the
inorganics, is slowly added to the organic medium. The viscosity
can be adjusted, if needed, by the addition of solvents. Mixing
methods that provide high shear are useful.
Formation of Electrodes
[0116] The paste can be deposited, for example, by screen-printing,
plating, extrusion, ink-jet printing, shaped or multiple printing,
or ribbons.
[0117] In this electrode-forming process, the paste is first dried
and then heated to remove the organic medium and sinter the
inorganic materials. The heating can be carried out in air or an
oxygen-containing atmosphere. This step is commonly referred to as
"firing." The firing temperature profile is typically set so as to
enable the burnout of organic binder materials from the dried paste
composition, as well as any other organic materials present. In one
embodiment, the firing temperature is 700 to 950.degree. C. The
firing can be conducted in a belt furnace using high transport
rates, for example, 100-500 cm/min, with resulting hold-up times of
0.03 to 5 minutes. Multiple temperature zones, for example 3 to 11
zones, can be used to control the desired thermal profile.
[0118] In one embodiment, a semiconductor device is manufactured
from an article comprising a junction-bearing semiconductor
substrate and a silicon nitride insulating film formed on a main
surface thereof. The instant paste is applied (e.g., coated or
screen-printed) onto the insulating film, in a predetermined shape
and thickness and at a predetermined position. The instant paste
has the ability to penetrate the insulating layer. Firing is then
carried out and the paste reacts with the insulating film and
penetrates the insulating film, thereby effecting electrical
contact with the silicon substrate and as a result the electrode is
formed.
[0119] An example of this method of forming the electrode is
described below in conjunction with FIGS. 1A-1F.
[0120] FIG. 1A shows a single crystal or multi-crystalline p-type
silicon substrate 10.
[0121] In FIG. 1B, an n-type diffusion layer 20 of the reverse
conductivity type is formed by the thermal diffusion of phosphorus
using phosphorus oxychloride as the phosphorus source. In the
absence of any particular modifications, the diffusion layer 20 is
formed over the entire surface of the silicon p-type substrate 10.
The depth of the diffusion layer can be varied by controlling the
diffusion temperature and time, and is generally formed in a
thickness range of about 0.3 to 0.5 microns. The n-type diffusion
layer may have a sheet resistivity of several tens of ohms per
square up to about 120 ohms per square.
[0122] After protecting the front surface of this diffusion layer
with a resist or the like, as shown in FIG. 1C the diffusion layer
20 is removed from the rest of the surfaces by etching so that it
remains only on the front surface. The resist is then removed using
an organic solvent or the like.
[0123] Then, as shown in FIG. 1D an insulating layer 30 which also
functions as an anti-reflection coating (ARC) is formed on the
n-type diffusion layer 20. The insulating layer is commonly silicon
nitride, but can also be a SiN.sub.x:H film (i.e., the insulating
film comprises hydrogen for passivation during subsequent firing
processing), a titanium oxide film, a silicon oxide film, or a
silicon oxide/titanium oxide film. A thickness of about 700 to 900
.ANG. of a silicon nitride film is suitable for a refractive index
of about 1.9 to 2.0. Deposition of the insulating layer 30 can be
by sputtering, chemical vapor deposition, or other methods.
[0124] Next, electrodes are formed. As shown in FIG. 1E, the thick
film paste of the present invention 500 is screen-printed to create
the front electrode on the insulating film 30 and then dried. In
addition, a back side silver or silver/aluminum paste 70, and an
aluminum paste 60 are then screen-printed onto the back side of the
substrate and successively dried. Firing is carried out in an
infrared belt furnace at a temperature range of approximately 750
to 950.degree. C. for a period of from several seconds to several
tens of minutes.
[0125] Consequently, as shown in FIG. 1F, during firing, aluminum
diffuses from the aluminum paste 60 into the silicon substrate 10
on the back side thereby forming a p+ layer 40 containing a high
concentration of aluminum dopant. This layer is generally called
the back surface field (BSF) layer, and helps to improve the energy
conversion efficiency of the solar cell.
[0126] Firing converts the dried aluminum paste 60 to an aluminum
back electrode 61. The back side silver or silver/aluminum paste 70
is fired at the same time, becoming a silver or silver/aluminum
back electrode, 71. During firing, the boundary between the back
side aluminum and the back side silver or silver/aluminum assumes
the state of an alloy, thereby achieving electrical connection.
Most areas of the back electrode are occupied by the aluminum
electrode 61, owing in part to the need to form a p+ layer 40.
Because soldering to an aluminum electrode is impossible, the
silver or silver/aluminum back electrode 71 is formed over portions
of the back side as an electrode for interconnecting solar cells by
means of copper ribbon or the like. In addition, the front side
thick film paste 500 of the present invention sinters and
penetrates through the insulating film 30 during firing, and
thereby achieves electrical contact with the n-type layer 20. This
type of process is generally called "fire through." The fired
electrode 501 of FIG. 1F clearly shows the result of the fire
through.
EXAMPLES
Solar Cell Electrical Measurements
[0127] A commercial Current-Voltage (JV) tester ST-1 000
(Telecom-STV Ltd., Moscow, Russia) was used to make efficiency and
fill factor measurements of the polycrystalline silicon
photovoltaic cells. Two electrical connections, one for voltage and
one for current, were made on the top and the bottom of each of the
photovoltaic cells. Transient photo-excitation was used to avoid
heating the silicon photovoltaic cells and to obtain JV curves
under standard temperature conditions (25.degree. C.). A flash lamp
with a spectral output similar to the solar spectrum illuminated
the photovoltaic cells from a vertical distance of 1 m. The lamp
power was held constant for 14 milliseconds. The intensity at the
sample surface, as calibrated against external solar cells was 1000
W/m.sup.2 (or 1 sun) during this time period. During the 14
milliseconds, the JV tester varied an artificial electrical load on
the sample from short circuit to open circuit. The JV tester
recorded the light-induced current through, and the voltage across,
the photovoltaic cells while the load changed over the stated range
of loads. A power versus voltage curve was obtained from this data
by taking the product of the current times the voltage at each
voltage level. The maximum of the power versus voltage curve was
taken as the characteristic output power of the solar cell for
calculating solar cell efficiency. This maximum power was divided
by the area of the sample to obtain the maximum power density at 1
Sun intensity. This was then divided by 1000 W/m.sup.2 of the input
intensity to obtain the efficiency which is then multiplied by 100
to present the result in percent efficiency. Other parameters of
interest were also obtained from this same current-voltage curve.
One such parameter is fill factor (FF) which is obtained by taking
the ratio of the maximum power from the solar cell to the product
of open circuit voltage and short circuit current. The FF is
defined as the ratio of the maximum power from the solar cell to
the product of V.sub.oc and I.sub.sc, multiplied by 100.
Comparative Experiment A
[0128] One batch of thick film paste was made by mixing 86.01 g
silver, 2.01 g Pb--Te--Li--O and 9.41 g organic medium in a plastic
jar using a THINKY.RTM. ARE-310 mixer (THINKY Corp., Laguna Hills,
Calif.) for 1 min at 2000 rpm. This step was repeated two more
times until a thoroughly mixed blend was obtained. The dispersed
mixture was then blended with triple roll mill (Charles Ross &
Son Company, Floor Model, 4''.times.8'') at a 1 mil gap for three
passes at zero psi and three passes at 100 psi to obtain a thick
paste. The viscosity of the final paste was measured using a
Brookfield HADV-1 Prime Viscometer (Brookfield Engineering Labs,
Inc., Middleboro, Mass.) with the thermostatted small-sample
adapter at about 10 rpm and was found to be 288 Pas. The solid
content of the final paste was calculated to be about 90.3 wt
%.
[0129] The Pb--Te--Li--O was prepared as follows. Mixtures of
TeO.sub.2 powder (99+% purity), PbO powder (ACS reagent grade, 99+%
purity) and Li.sub.2CO.sub.3 in the % cation ratio Te:Pb:Li of
57:38:5 were tumbled in a polyethylene container for 30 min to mix
the starting powders. The starting powder mixture was placed in a
platinum crucible and heated in air at a heating rate of 10.degree.
C./min to 900.degree. C. and then held at 900.degree. C. for one h
to melt the mixture. The melt was quenched from 900.degree. C. by
removing the platinum crucible from the furnace and pouring the
melt onto a stainless steel platen. The resulting material was
ground in a mortar and pestle to less than 100 mesh. The ground
material was then ball-milled in a polyethylene container with
zirconia balls and isopropyl alcohol until the average particle
size (d.sub.50) was 0.5-0.7 microns. The ball-milled material was
then separated from the milling balls, dried, and run through a 100
mesh screen to provide the PbO--TeO.sub.2--Li.sub.2O powder
(Pb--Te--Li--O) used in the thick film paste preparations.
[0130] The organic medium components of the thick film paste and
the quantities used are given in Table 5. The organic medium was
prepared by mixing the components.
TABLE-US-00005 TABLE 5 Component Wt. (g) Ethyl Cellulose (50-52%
ethoxy) 1.30 Ethyl Cellulose (48-50% ethoxyl) 0.51 Duomeen .RTM.,
an amine oleate surfactant 1.09 Foralyn .TM. (hydrogenated rosin
ester) 2.51 Thixatrol .RTM. ST, hydrogenated castor oil 0.50 DBE-3
(dibasic ester-3) 3.50
[0131] The resulting paste was used in Comparative Experiment A.
The paste was printed on the front side of a 28 mm.times.28 mm
multicrystalline silicon wafer 200 .mu.m thick to form a front side
electrode and then dried. A commercially available aluminum paste
PV381 (DuPont Co., Wilmington, Del.) was screen printed on the back
side of the silicon wafer to form the back side electrode and then
dried. Essentially identical additional wafers were prepared in a
similar fashion. These wafers were fired at four different maximum
firing temperatures ranging from 910 to 955.degree. C. to form the
front and back side electrodes of the solar cell. At least three to
five wafers were fired at each of the temperatures. The median
solar cell efficiency and the fill factor (FF), measured as
described above, are shown in Table 6 for the samples prepared at
each of the maximum firing temperatures. Since the paste made and
used in this Comparative Experiment contained Ag but no copper, it
is designated as having a composition of 100 vol % Ag, based on the
total volume of Ag and copper.
Example 1
[0132] A second batch of thick film paste was made essentially as
described in Comparative Experiment A, except that the amount of
Ag, i.e., the volume of Ag, was replaced with a mixture of Ag and
copper in which 75% of that volume was Ag and 25% was fatty acid
coated copper. The fatty acid coated copper contained a 0.1% fatty
acid coating on copper with a median particle size d.sub.50 of 3.3
.mu.m (lot #7032-2 obtained from CuLox Technologies, Inc.,
Naugatuck, Conn.). This paste is designated as having a composition
of 75 vol % Ag/25 vol % fatty acid coated copper (Cu), based on the
total volume of Ag and copper. Solar cells were made and
measurements obtained as described in Comparative Experiment A. The
median solar cell efficiency and the fill factor, measured as
described above, are shown in Table 6 for the samples prepared at
each of the maximum firing temperatures.
Example 2
[0133] A third batch of thick film paste was made essentially as
described in Comparative Experiment A, except that the amount of
Ag, i.e., the volume of Ag, was replaced with a mixture of Ag and
copper in which 75% of that volume was Ag and 25% was tin (Sn)
coated copper in which the tin comprised 1.25 vol %. The tin coated
copper had a median particle size d.sub.50 of 13 .mu.m (lot #3205-2
obtained from Technic, Inc., Cranston, R.I.). This paste is
designated as having a composition of 75 vol % Ag/25 vol % Sn
coated copper, based on the total volume of Ag and Cu. Solar cells
were made and measurements obtained as described in Comparative
Experiment A. The median solar cell efficiency and the fill factor,
measured as described above, are shown in Table 6 for the samples
prepared at each of the maximum firing temperatures.
Example 3
[0134] A forth batch of thick film paste was made essentially as
described in Comparative Experiment A, except that the amount of
Ag, i.e., the volume of Ag, was replaced with a mixture of Ag and
copper in which 75% of that volume was Ag and 25% was copper. The
copper had a median particle size d.sub.50 of 4.3 .mu.m (lot
#56109150802 obtained from ACuPowder International, LLC, Union,
N.J.). This paste is designated as having a composition of 75 vol %
Ag/25 vol % Cu, based on the total volume of Ag and Cu. Solar cells
were made and measurements obtained as described in Comparative
Experiment A. The median solar cell efficiency and the fill factor,
measured as described above, are shown in Table 6 for the samples
prepared at each of the maximum firing temperatures.
TABLE-US-00006 TABLE 6 Paste Maximum Efficiency FF Composition
Firing Temp. (%) (%) 100 vol % Ag 910.degree. C. 15.16 77.7
(Comparative 925.degree. C. 14.56 76.3 Experiment A) 940.degree. C.
15.47 77.4 955.degree. C. 15.25 76.5 75 vol % Ag/ 910.degree. C.
11.43 59.5 25 vol % (fatty 925.degree. C. 14.72 75.1 acid coated)
Cu 940.degree. C. 14.88 76.6 (Example 1) 955.degree. C. 14.29 74.9
75 vol % Ag/ 910.degree. C. 10.87 55.4 25 vol % (Sn 925.degree. C.
13.10 67.3 coated) Cu 940.degree. C. 14.52 74.0 (Example 2)
955.degree. C. 15.0 77.5 75 vol % Ag/ 910.degree. C. 10.98 57.3 25
vol % Cu 925.degree. C. 12.23 63.55 (Example 3) 940.degree. C.
13.71 72.85 955.degree. C. 13.70 73.30
* * * * *